Integration Question

I would like some clarification on how aMember works with third party databases. I'm currently building a website that will be made using Expression Engine and an Invision Power Board, and I'd like to use aMember as a bridge between the two. Since I'm building the site for myself, not a client, I have a small budget to work with at the beginning. So, if I were to use only the IPB login/register for members during the first few months, how easy would it be to integrate the database with aMember when I purchase it later so that I have a single login between EE and IPB? I don't really have an intention to use aMember for much more than a bridge.

I read the page on integration, but the wording was a bit unclear to me.